Membrane-bound cholesterol in lymphocytes can be manipulated by treating these cells with oxygenated derivatives of cholesterol, which specifically depress the de novo synthesis of cholesterol. Inhibition of cholesterol synthesis in lymphocytes abolishes their DNA synthesis and thus interferes with cell division. In addition, specific immunological functions of T-lymphocytes, such as target cell killing, are abolished. We propose to analyze the mechanisms through which membrane-bound cholesterol is involved in T-cell killing. The importance of de novo synthesis of cholesterol for the induction of a secondary, cell mediated response will be analyzed in detail. These studies will be concerned also particularly with the sterol and phospholipid composition of the plasma membrane of immunocompetent cells. Some sera used for tissue culture purposes are inhibitory to lymphocytes and prevent proliferation either induced by lectins or by an allogenic lymphocyte reaction. We propose to investigate whether the inhibitory activity of such sera resides in the lipids, specifically in sterols of the lipoprotein fraction. The role of membrane bound cholesterol for antibody production and secretion will be investigated in MOPC-104E plasmocytoma cells as well as in B-lymphocytes stimulated by lipopolysaccharide (LPS). A series of experiments is also directly concerned with the structure of the LPS receptor on B-cells and its dependence on the lipid composition of the plasma membrane. The regulation of the synthesis of cholesterol in lymphocytes and leukemic cells will be investigated by studying the structural requirements of the inhibitory sterol molecules and by examining the binding of non-inhibitory sterols to cytoplasmic receptors in normal and malignant lymphocytes. The effects of oxygenated sterols such as 25-hydroxycholesterol or 7-ketocholesterol, administered to animals in vivo, through the diet or intraperitoneally, will be analyzed. We will especially focus on the effects of such treatments on the cell mediated and humoral immune response in the intact animal as well as on their effects upon the growth of transplanted and spontaneous tumors.